Move code for managing PartitionDescs into a new file, partdesc.c

This is similar in spirit to the existing partbounds.c file in the
same directory, except that there's a lot less code in the new file
created by this commit.  Pending work in this area proposes to add a
bunch more code related to PartitionDescs, though, and this will give
us a good place to put it.

Discussion: http://postgr.es/m/CA+TgmoZUwPf_uanjF==gTGBMJrn8uCq52XYvAEorNkLrUdoawg@mail.gmail.com

src/include/partitioning/partdesc.h

@@ -0,0 +1,39 @@
+/*-------------------------------------------------------------------------
+ *
+ * partdesc.h
+ *
+ * Copyright (c) 1996-2018, PostgreSQL Global Development Group
+ *
+ * src/include/utils/partdesc.h
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef PARTDESC_H
+#define PARTDESC_H
+
+#include "partitioning/partdefs.h"
+#include "utils/relcache.h"
+
+/*
+ * Information about partitions of a partitioned table.
+ */
+typedef struct PartitionDescData
+{
+	int			nparts;			/* Number of partitions */
+	Oid		   *oids;			/* Array of 'nparts' elements containing
+								 * partition OIDs in order of the their bounds */
+	bool	   *is_leaf;		/* Array of 'nparts' elements storing whether
+								 * the corresponding 'oids' element belongs to
+								 * a leaf partition or not */
+	PartitionBoundInfo boundinfo;	/* collection of partition bounds */
+} PartitionDescData;
+
+extern void RelationBuildPartitionDesc(Relation rel);
+
+extern Oid	get_default_oid_from_partdesc(PartitionDesc partdesc);
+
+extern bool equalPartitionDescs(PartitionKey key, PartitionDesc partdesc1,
+					PartitionDesc partdesc2);
+
+#endif							/* PARTCACHE_H */